Darth Sidious is also known as Emperor Palpatine and is the main antagonist in both Star Wars Trilogies Episodes I through VI.

Real Name: Palpatine

Alias: Darth Sidious, Supreme Chancellor of the Republic, Emperor of the Empire

Species: Human

Home Planet: Naboo

Born: 82 BBY

Death: 4 ABY

Apprentices: Darth Maul, Darth Vader, Darth Tyranus

Canon: Darth Sidious / Emperor Palpatine is a main character in Episodes I-VI.

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Training and Rise of the Empire
Palpatine is born to a powerful family on the planet of Naboo. He became fascinated with the dark side of the force at a young age. He found and began collecting Sith artifacts and marveled at their mysterious history and enigmatic power. He would soon come into contact with a real Sith Lord and go down a path of becoming one himself.

Darth
Plagueis begins to work to acquire valuable resources from the planet
of Naboo. His company along with the Trade Federation are being
thwarted by isolationist forces on the planet. The consortium finds an
ally in Palpatine who opposes his isolationist father. Here Palpatine sees an opportunity to gain a powerful ally and training in the darkside.

Palpatine
begins to feed information to Hego Damask (Darth Plagueis) in exchange
for political and financial powers of Damask Holdings. The two have a
growing father and son relationship which disturbs Palpatine's father
who attempts to intercede. Palpatine begins to be trained by Darth
Plagueis in ways of the dark side of the force. Palpatine uses these
new force powers to kill off his entire family. With this action he is
congratulated by Hego Damask and becomes the official apprentice of
Darth Plagueis. He takes on the name, Darth Sidious.

Palpatine being raised to Supreme Chancellor by Senate

Plagueis
began to show his ability of bringing others back from death. He was
unable to do this for himself. He showed Sidious this by killing Darth
Venamis and bringing him back to life. While powerful Darth Plagueis
began to fear that the Force could strike back at him for using his life
altering powers. He became less interested in acquiring powers and
more fearful about losing it.

During this time Sidious begin to train his own apprentice Darth Maul. He also begins to develop a relationship with the Jedi Count Dooku who will later become Darth Tyranus.

While Plagueis is focused on his force studies. Palpatine uses his Sith abilities and political ambition to create a war. Playing both sides of the fray he is elevated to a dictatorial like position by the Republic's senate. This war between the Republic and Separatist is known as the Clone Wars due to the clone armies used by the Republic. Palpatine effectively controls both sides of the conflict and uses it as a tool to spread fear throughout the galaxy and gain power.

Around the time of Darth Sidious's coronation to Vice Chancellor he felt that Plagueis had
used up his usefulness, he lulled the old Sith master to sleep and killed
him.Palpatine uses his new powers to dissolve the Republic and replace it with an Empire with himself at its helm. He uses a code embeded in the minds of the clone troopers called Order 66 that causes them to turn on their Jedi Masters. Palpatine meanwhile lures his knew apprentice Anakin Skywalker to the dark side. Anakin helps Palpatine survive an assualt from Jedi Master Mace Windu This battle ends in Windu's death. Palpatine then has Anakin execute the remaining Jedi at the Jedi Temple turning him fully to the darkside. Anakin then takes on the name of Darth Vader and becomes Palpatine loyal servant until the Battle of Endor.

As Emperor Palpatine extends the reach of the Empire and uses its armies to take over vast amounts of territories. The Empire is met with the resistance from a group known as the rebellion. To end the threat of the rebellion Palpatine begins to build a massive weapon known as the Death Star that can destroy planets. The first deathstar is destroyed by a force sensitive pilot known as Luke Skywalker. Realizing that Luke is the son of Anakin, Palpatine orders Vader to track him down and turn him toward the darkside.

Palpatine plunges to his demise

The Emperor is killed in an attempt to turn Luke Skywalker. Darth Vader betrays him and throws him off a massive pit in Death Star II. Death Star II is later destroyed. It is assumed that Palpatine was killed either from the fall or from the explosion of the Death Star.Star Wars Speculation:

Is Darth Sidious really Supreme Leader Snoke?
Plagueis is a heavily rumored candidate to be Snoke.

The reasons to believe Plagueis is Snoke are numerous.

Sidious somewhat resembles Plagueis

It is possible he survived the fall

What makes this unlikely is that Palpatine would most likely continue with his name as Emperor and use his mastery over death to boost his own legend.